At present, plastic containers (e.g., bottles, etc.) composed mainly of a polyester, such as polyethylene terephthalate (PET), etc., are widely used for teas, fruit juice beverages, carbonated beverages, and the like. In addition, the share of small-sized plastic bottles among plastic containers is increasing year by year. Following downsizing of the bottle, a ratio of the surface area per unit volume becomes large, and therefore, when downsizing the bottle, a shelf life of the contents tends to become short. In addition, recently, the application of the plastic container has been broadened, for example, by the sale of beer, which is susceptible to oxygen or light, being packaged in a plastic bottle, or the sale of a hot tea packaged in a plastic bottle. Under such a circumstance, a more enhancement in gas barrier properties against the plastic container is required.
In order to cope with the aforementioned requirement, as for a method of giving gas barrier properties to the bottle, there have been developed multilayer bottles comprising a polyester resin and a gas barrier resin; blend bottles; barrier-coating bottles produced by forming a carbon coat, a deposited coat, or a barrier resin coat onto a single-layer bottle made of a polyester resin; and the like.
In multilayer bottles, it is known that a thermoplastic polyester resin, such as PET, etc., is used for materials of an innermost layer and an outermost layer, and a thermoplastic gas barrier resin, such as polymetaxylyleneadipamide (polyamide MXD6), etc., is used for an interlayer. It is known that the multilayer bottle is obtained by preparing a preform (parison) having a three-layer or five-layer structure, which is obtained by filling a die cavity by injection molding these resins, and then subjecting the preform to biaxial stretch blow molding.
The aforementioned multilayer bottles are utilized for containers for beer, teas, carbonated beverages, and the like because of their good gas barrier properties. When the multilayer bottles are used in these applications, the quality of the contents filled therein can be maintain and the shelf life can be improved. On the other hand, there is a concern that the multilayer bottles undergo delamination between different resin layers, for example, between the innermost or outermost layer and the interlayer, resulting in damaging their commercial value.
As a method of improving such a problem, Patent Document 1 discloses a method in which when a resin constituting the innermost or outermost layer is injected into a die cavity at the end, a fixed amount of the resin constituting the innermost or outermost layer is allowed to flow back toward the side of the gas barrier layer. According to this method, the mixed resin enters between the layers, thereby improving the delamination resistance. However, the method described in Patent Document 1 involves such a problem that use of a special apparatus is required in order to allow the resin to flow back.
In addition, as other solution, there is known a method in which an interlayer is formulated with a blend of a polyester resin, such as PET, etc., and polyamide MXD6, to increase an affinity between the polyester resin as a contact layer and the interlayer, thereby improving the delamination resistance (see Patent Document 2). This method is advantageous from the standpoint of costs because a large modification of a molding apparatus is not required.
However, as compared with the case where the formulation of the interlayer is made of only polyamide MXD6, in this method, the heat stability of the resin is reduced more than expected, and the resin remains in the inside of a molding machine for a long period of time when molding a bottle, and thus, the deteriorated matter becomes scorch and contaminates the product, resulting in deteriorating the product yield. Accordingly, it becomes necessary to use a purging agent during the production, or to remove the deteriorated matter by, for example, disassembling and cleaning the die, and therefore, the stable production is disturbed. In addition, the scorch sometimes plugs a passage of the inside of the molding machine, thereby making it impossible to undergo molding.
In addition, conventionally, in order to prevent deterioration of polyamide resins, the apparatus is improved to minimize a remaining portion in the inside of the molding machine, or plating is conducted. This can suppress the formation of a deteriorated matter, but does not completely solve the problem.
Meanwhile, a variety of methods of suppressing the generation of a fish eye or a gel at the time of molding by improving materials in various ways are devised (see Patent Documents 4 to 9). However, any useful method of suppressing the generation of scorch at the time of bottle molding by improving materials has not hitherto been found.